I would be grateful if temperature delta's of GPU and mem are known at full power vs idle
with 35C on the mainboard idle
There is no ideal idle temp. Since idle temp vastly differs by ambient temp, cooling solution, fan RPM, fan size and fan amount. (Dust too.)
Hence why, there are no idle temp charts in component reviews. Only under load ones.
E.g RTX 5070 review temp chart:
https://www.techpowerup.com/review/nvidia-geforce-rtx-5070-founders-edition/41.html
Note: You will never get lower temps than ambient temps. Since PCs are cooled by ambient air.
Also, idle temps doesn't matter because even the smallest cooler can keep CPU/GPU near ambient temps. What matters, is under load temps, since there you can see the effectiveness of your cooling solution (usually balance between noise and temperature target).
E.g in my Skylake build (full specs with pics in my sig), idle temps currently are:
* 27.2C - ambient
* 30C - CPU (i5-6600K)
* 42.7C - GPU (GTX 1660 Ti)
* 41C - MoBo chipset (Z170)
* 49C - OS drive (970 Evo Plus 2TB)
* 35C - OS backup drive (960 Evo 500GB)
* 33C - data drive (870 Evo 2TB)
* 36C - data backup drive (MX500 1TB)
But these values of mine will tell you 0, since you do not have the same ambient, components and airflow setup as i have.
In the review, RTX 5070 temp delta is up to 35C. Not that it would matter, since GPU would remain under 80C under load.
Now, if you're building mini-ITX system with few or no case fans, GPU (and CPU too) can be cooked into thermal throttle.